Bacteria
all have in common the prokaryotic cell type, but there are many groups and
many ways of classifying them. Bergey’s Manual is
the most widely accepted authority.
Bacteria make up two of the three domains—the domain Bacteria (the
common everyday bacteria) and the domain Archaea (strange and unusual
bacteria).
They are
subdivided into various groups according to Gram stain reaction, cell shape,
cell arrangements, oxygen requirements, motility, and nutritional and metabolic
properties.
Table 11.1 P. 314 – 316 gives a summary of the various groups, including major genera
and special features. Most of the remaining chapter covers this same
information, but in more detail. Most of the bacteria mentioned are those of
practical importance, medical importance, etc. Many genera are omitted if they do not fit
this description.
Only a
small percentage of bacterial genera have been discovered and classified. It is
estimated that only 5 – 10 % of bacteria are potentially harmful. The rest are
harmless or beneficial.
1. ALPHA-PROTEOBACTERIA (a-PROTEOBACTERIA)—this group
includes most of the proteobacteria
that can grow with low levels of nutrients.
·
Some have unusual
morphology
·
Nitrogen-fixing bacteria
are important in agriculture
·
Several plant and human
pathogens are included
·
All are gram-negative
·
The majority are rods,
although other forms are also found
a. Azospirillum—organisms
of this genus grow in soil in close association with the roots of certain
plants such as sugar cane, corn, and tropical grasses. These organisms fix nitrogen.
b. Acetobacter and Gluconobacter
both are used in industry to convert ethanol into vinegar (acetic acid)
c. Rickettsia—rods or coccobacilli
which are obligate intracellular parasites (reproduce only inside a mammalian
cell). Transmitted to humans by the bites of insects or
ticks. Cells take the bacteria in by phagocytosis and reproduction
occurs inside the cell by binary fission. Various rickettsias
cause a group of
diseases known as the spotted fevers—all involve a rash due to
damage to capillaries. Dr. Howard Ricketts, who discovered these organisms and
gave them his name,
died after contracting a rickettsial
disease in a lab accident, proving that these can be quite dangerous.
1) Rickettsia prowazekii causes epidemic typhus fever,
transmitted by lice
2) Rickettsia typhi causes endemic murine typhus,
transmitted by rat fleas
3) Rickettsia rickettsii causes Rocky Mountain Spotted Fever, transmitted by ticks
d. Ehrlichia species live within white blood cells and are similar to the rickettsias. They are transmitted by ticks to humans and
cause ehrlichiosis, a serious blood disease.
e. Caulobacter and Hyphomicrobium—both live in water and produce
stalk-like structures called prosthecae to anchor
themselves to surfaces. Hyphomicrobiumr reproduces by budding.
f. Rhizobium, Bradyrhizobium and Agrobacterium
1) Rhizobium & Bradyrhizobium—live in nodules in
the roots of legumes (beans, peas, clover) and carry on nitrogen fixation
2) Agrobacterium—this genus includes
a plant pathogen which causes crown gall. (The crown is the area where roots
and stem merge; a gall is a tumor-like growth).
g. Bartonella—B.
henselae causes cat-scratch disease
h. Brucella—small nonmotile coccobacillus—all species are obligate parasites. The
disease is brucellosis. These organisms can survive phagocytosis.
i. Nitrobacter and Nitrosomonas—both
are nitrifying bacteria which use reduced nitrogen compounds for energy and
deposit nitrate into the soil. These are chemoautotophs.
j. Wolbachia—live
only inside the cells of their insect hosts
2. BETA-PROTEOBACTERIA (b-PROTEOBACTERIA)—often use nutrient substances that
diffuse away from decomposing organic matter but several pathogens are in this
group
a. Thiobacillus—use sulfur in energy
production
b. Spirillum—spiral
shape, live in water and are motile by polar flagella. They are relatively
large and aerobic.
c. Sphaerotilus—sheathed bacteria
found in fresh water and sewage. They form a hollow filamentous sheath and live
inside. This can clog sewage plants.
d. Burkholderia—this is a recently
classified genus, split off from Pseudomonas.
Organisms are motile by flagella and aerobic. B. cepacia can break down a great variety
of organic compounds and may be a problem in contamination of medical equipment
in hospitals. The organism is also found causing infections, particularly in
cystic fibrosis patients.
e. Bordatella—B. pertussis is an aerobic rod which causes whooping cough
f. Neisseria—these aerobic cocci usually inhabit the mucous membranes
of mammals. Some of them are harmless, but two major pathogens are included:
1) Neisseria gonorrhoeae—causative agent of
gonorrhea
2) Neisseria
meningitidis—causative agent of meningococcal meningitis
g. Zooglea—beneficial in aerobic
sewage treatment plants
3. GAMMA-PROTEOBACTERIA (g-PROTEOBACTERIA) --some
names in this group are order names. Up to now the names have been genus
names. If it is in italics, it is a genus.
If it is in all caps but not in bold, it is an order. The all caps in bold are
groups larger than orders.(If you think this is
confusing, I do too, but this is what our book does!)
a. Beggiatoa alba—only species in the genus. Grows
in aquatic sediments and is motile by gliding. It uses hydrogen sulfide for
energy and stores sulfur granules as cellular inclusions.
b. Francisella—small, pleomorphic
bacteria that require media enriched with blood or tissue extracts. F. tularensis causes tularemia (rabbit fever).
c.
PSEUDOMONALES (an order)—all are gram-negative and aerobic. Shape is rod or coccus.
1) Pseudomonas—aerobic rods, motile by
flagella. Many produce pigments which diffuse into their surroundings.
a) P. aeruginosa—produces blue-green pigment and commonly causes infections of the
urinary tract, burns and wounds. The pus has the blue-green color and a
horrible odor. This organism is a big problem in
hospitals due to its resistance to many antibiotics and the ability to grow on
equipment and even in some solutions of disinfectants.
b) P. syringae—occasional plant pathogen
c) In general, pseudomonads are often found in soil and may decompose
chemicals there (breaking down harmful substances such as pesticides
is beneficial). They also are associated with food spoilage. Members of
this group are able to produce an unusually large number of enzymes.
2) Azotobacter and Azomonas—free-living
in soil. These are capable of nitrogen fixation but often fix little because
other nutrients they require are in short supply in soil.
3) Moraxella—aerobic
coccobacilli. M. lacunata causes
conjunctivitis (pinkeye).
d. LEGIONELLALES—another order.
1) Legionella—this entire genus was discovered fairly
recently. They have probably been around longer than humans, but we had never
found them. In 1976
there was an outbreak of an unusual pneumonia in persons who had attended a
national convention of the American Legion.
These bacteria were eventually found to be the cause. The difficulty was
that they will not grow on normal lab media, but media which meet their requirements
were developed and now several species have been identified. In nature they
live in water. Occasionally they colonize water lines, air conditioning
systems, produce misting systems, etc. and cause disease when they are inhaled.
2) Coxiella –C. burnetii
causes a disease called Q fever. It was first grouped with the rickettsias, but this one is different because it is not
transmitted by insects. Coxiella is like the rickettsias
in that it can only reproduce inside a mammalian cell. It is often associated
with cows and is transmitted to humans through aerosols or contaminated milk.
(Pasteurization kills it.)
e. VIBRIONALES—this order
consists of facultatively anaerobic rods which are usually slightly curved
1) Vibrio is the main genus. It includes V. cholerae,
the causative agent of cholera. This disease once caused great epidemics with a
high death rate due to extreme diarrhea. V.
parahaemolyticus is associated with eating raw or
undercooked shellfish and causes a less severe gastroenteritis.
f.
ENTEROBACTERIALES—an order
·
All are gram-negative
facultatively anaerobic rods
·
Simple nutritional
requirements
·
Many are motile by peritrichous flagella
·
Commonly known as the enterics, because they live in the intestinal tracts of
humans and other animals
·
Many are part of what is
known as the normal flora of the body (microbes that are always present and
usually harmless)
·
A few members of this group
are found only as potential pathogens (disease-causing organisms)
·
Even the normal flora
organisms can become pathogens in unusual circumstances
·
All enterics
are quite similar so specific identification can be quite difficult, often
requiring biochemical tests and serology
·
Enterics have fimbriae, which they use to attach to surfaces, especially
the mucous membranes of their normal environment.
·
They also have sex pili,
which they use to transfer genetic information between cells. This is bad news
for humans, because antibiotic resistance is often passed between organisms
this way
·
Enterics produce proteins called bacteriocins,
which are harmless to the species producing them but cause lysis of other
species in the environment. This probably helps maintain the balance between
the normal flora species and also helps eliminate pathogens in the intestines
1) Escherichia—E. coli is a
major member of the enteric group. It is also the most studied and familiar
bacterial species used in biological research. Tests to detect its presence in
water or food are important because this is considered an indication of fecal
contamination. Although regular strains of E.
coli are normal and harmless in the intestinal tract, there are exceptions:
·
Regular strains cause many urinary tract
infections—they are OK in the intestines but pathogenic in other locations
·
Certain strains produce enterotoxins,
which cause travelers' diarrhea
·
One specific strain, E. coli O157:H7 produces a
toxin which causes a severe or fatal foodborne
disease
2) Salmonella—these enterics are not normal flora, and their presence in the intestinal tract of humans
is not considered normal. Animals, especially poultry and cattle, frequently do
carry these organisms, and they can contaminate food. Nomenclature and
classification of these organisms does not follow the usual pattern. All
salmonellas are a single species, Salmonella
enterica. The species is then divided into over
2400 serovars (serotypes) ,
meaning their serological reactions differ, which allows identification. Even
though it is not really following correct procedure, the common serovars are given names and often written as if they were
species names—
Salmonella typhi or Salmonella cholerasuis
for example. The really correct way
would be Salmonella enterica
serovar Typhi.
Salmonella typhi causes
typhoid fever, the most severe illness associated with this genus. All others
can cause salmonellosis, a very common foodborne
illness.
3) Shigella—species cause shigellosis, also called bacillary dysentery.
This can be quite severe, especially in babies and the elderly. This genus is
found only in humans.
4) Klebsiella—live in soil and water
as well as intestinal tracts. Many can fix nitrogen. K. pneumoniae can cause
severe pneumonia, usually in patients in a very poor nutritional state.
5) Serratia—one species, S. marcescens, produces a reddish-orange pigment. It is a
common contaminant in hospitals, frequently found on catheters and in
supposedly sterile solutions. It has been know to establish itself in air ducts
and cause pneumonia due to inhalation. It also may cause urinary tract
infections.
6) Proteus—this genus shows a
swarming growth on agar—this means it spreads all over the place. May cause
urinary tract infections and infect wounds.
7) Yersinia---Y. pestis
causes bubonic plague—the Black Death of medieval times. Rats and wild rodents
may carry this organism, and it spreads to humans by way of fleas. Once
infection occurs, it can pass from person to person also. This is not only a
disease from way back in history—there are usually a few cases in the
8) Erwinia—this genus differs from
the rest. It is primarily a pathogen of plants, causing rot.
9) Enterobacter—these can live in
water and soil as well as intestinal tracts. E. cloacae and E. aerogenes can cause urinary tract infections and nosocomial (hospital-acquired) infections.
g. PASTEURELLALES—this order
includes human and animal pathogens. The organisms are nonmotile.
1) Pasteurella—mainly animal
pathogens which can cause pneumonia and septicemia (bacteria living and
multiplying in the blood—a very serious situation). P. multocida can be transmitted from dogs and cats to
humans by bites or wounds.
2) Haemophilus—found in the mucous
membranes of the upper respiratory tract, the mouth, the vagina and the
intestinal tract. H. influenzae
got its name from the mistaken belief that it caused influenza. It is a common
cause of ear infections and sometimes causes meningitis in children. It can
also cause epiglottitis, septic arthritis,
bronchitis, and pneumonia. H. ducreyi causes a sexually transmitted disease called chancroid. When grown in the lab, these organisms require
blood in their culture medium.
4. DELTA-PROTEOBACTERIA (d-PROTEOBACTERIA)—includes bacteria that prey on other bacteria and
also contributors to the sulfur cycle.
a) Bdellovibrio is the main genus.
There attach to other gram-negative bacteria and penetrate into the periplasmic space (between outer membrane and plasma
membrane). The original invader elongates and breaks into pieces, producing
several new cells. The host cell lyses and releases them.
b) DESULFOVIBRIONALES—this order of obligate
anaerobes use oxidized sulfur as terminal electron acceptors, producing
hydrogen sulfide which is released into the atmosphere.
1) Desulfovibrio is the best-studied genus.
2) Beggiatoa is another genus in this group.
c)
MYXOCOCCALES--fruiting and gliding bacteria. The geuns
Myxococcus produces
vegetative cells that move by gliding, leaving a slime trail. They feed on
other bacteria. Large numbers aggregate (gather) and form a macroscopic
fruiting body containing resting cells called myxospores.
Later these myxospores germinate and form vegetative
gliding cells.
5. EPSILON-PROTEOBACTERIA (e-PROTEOBACTERIA)—slender rods, helical to vibroid. Microaerophilic and motile by flagella.
a) Campylobacter—vibroid with one polar
flagellum
1) Campylobacter fetus—spontaneous abortions in domestic animals
2) Campylobacter jejuni—foodborne intestinal disease (diarrhea)
b) Helicobacter—curved rods with multiple
flagella. H. pylori is the cause of
most peptic ulcers in humans and can even be involved in stomach cancer .
B. NONPROTEOBACTERIA---GRAM
NEGATIVE BACTERIA---these are only distant relatives of the gram-negative proteobacteria.
1. Cyanobacteria—these
have a blue-green color and resemble algae. They carry out oxygenic
photosynthesis and many of them fix nitrogen. Special cells called heterocysts contain enzymes that convert nitrogen gas to
ammonium ions. Most species grow in water and have gas vacuoles that allow the
cells to float. Some are unicellular and divide by binary fission. Other forms
are colonies and filamentous forms. Cyanobacteria are
believed to have played a big role in bringing oxygen levels up in our
atmosphere and they still add oxygen as well as fixing nitrogen.
2.
Purple and Green Photosynthetic Bacteria--this is not really an
independent group. The photosynthetic bacteria are found scattered in various
other groups.
Purple sulfur bacteria—gamma-proteobacteria
Purple nonsulfur
bacteria—alpha-proteobacteria
Green sulfur bacteria—non-proteobacteria
Green nonsulfur
bacteria—non-proteobacteria
In
general, they are mostly anaerobic and live in sediments of lakes and ponds.
Shape may be spirals, rods, or cocci. They carry out anoxygenic photosynthesis.
The following overall reaction occurs in purple and green sulfur bacteria:
2 H2S
+ CO2 à (CH2O) + H2O + 2 S
Purple and
green nonsulfur bacteria reduce CO2
using organic compounds such as acids or carbohydrates as electron
donors.
II.
GRAM-POSITIVE BACTERIA
These are divided into two
groups---those that have a high G + C ratio and those with a low G + C ratio.
(To determine this, the DNA of the organisms is studied. The percentage of
cytosine and guanine is compared to the percentage of adenine and thymine and
that is the G + C ratio.)
A. LOW
G + C GRAM-POSITIVE
BACTERIA----the Firmicutes
1.
CLOSTRIDIALES
a. Genus Clostridium—these are obligate anaerobes that form endospores,
which are highly resistant to heat, drying, and chemicals.
1) C. tetani—causative agent of tetanus
2) C. botulinum—causes
botulism, a severe form of food poisoning that can be fatal
3) C. perfringens—gas gangrene in wounds and also gastrointestinal
upsets
4) C. difficile—can cause very serious
diarrhea (pseudomembranous colitis) when other
inhabitants are killed off by antibiotics.
b. Epulopiscium—E. fishelsoni is newly discovered and
really strange. Its size is tremendous compared to other bacteria—80 X 600 mm when normal bacteria might be 1
X 2 mm. It was
found living in symbiosis in the gut of
2. BACILLALES—gram-positive
rods and cocci.
a. Bacillus—these are rods that produce
endospores. Most are nonpathogenic but there are exceptions:
1) Bacillus anthracis causes anthrax.
Although this is primarily a disease of cattle, sheep and horses, it can be
transmitted to humans. This organism is a large gram-positive rod, nonmotile
and facultatively anaerobic. Endospores are centrally located. Since endospores
are highly resistant to drying and anthrax endospores are infectious by
inhalation, this organism has great potential as an agent of biological
warfare.
2) Bacillus thuringiensis is a microbial
insect pathogen. Endospores or crystallized toxins of this organism are used as
a spray on plants. When ingested by an insect, the toxin causes paralysis of
the gut and the insect dies. In some insects the endospores germinate and
bacteria contribute to the insect’s death. Bacteria normally found associated
with plant roots have been genetically engineered to produce the same toxin so
they can also kill insects feeding on the plant.
3) B. cereus—common in the environment, occasionally causes food
poisoning associated with starchy foods such as rice.
3. Staphylococcus--these organisms are gram-positive cocci which
usually occur in grapelike clusters. Although many species of this genus exist
as harmless inhabitants of skin and mucous membranes, one member, Staphylococcus aureus, is capable of
causing severe infections that are often resistant to many antibiotics. Some,
in fact, are resistant to all known antibiotics currently available. Both
pathogenic and nonpathogenic species are able to grow well in surroundings of
fairly high osmotic pressure, which allows them to grow in nasal secretions. It
is not uncommon to find that antibiotic-resistant Staphylococcus organisms have colonized the noses of hospital
workers, usually without symptoms, and eliminating these bacteria can be quite
difficult. Staphylococcus also is
able to grow in foods such as ham, where osmotic pressure prevents many other
bacteria from surviving.
Staphylococcus aureus
produces a variety of toxins, which increase its ability to spread and invade
tissue. Infections of surgical or other wounds with this organism is a common
problem, and it can cause fatal nosocomial
infections. One strain produces a toxin
that causes toxic shock syndrome. The organism also produces an enterotoxin
that causes food poisoning. This toxin is not destroyed by normal reheating of
food.
4. LACTOBACILLALES
a. Lactobacillus—although these organisms are unable to use oxygen,
they are aerotolerant. They ferment carbohydrates and produce large amounts of
lactic acid as an end product. The resulting acidity in their surroundings
inhibits growth of many other types of competing bacteria. Humans normally have
lactobacilli in the vagina, intestinal tract, and the mouth. These harmless
bacteria play a major role in limiting the growth of potentially harmful
species. In fact, during or following antibiotic treatment, freeze-dried
lactobacilli are sometimes used to help reestablish normal balance in the
intestine.
Lactobacilli
also are responsible for production of sauerkraut, pickles, buttermilk, and
yogurt.
b. Streptococcus--these are gram-positive cocci, which are likely to
grow in chains of varying length. They are mostly aerotolerant, although a few are
anaerobic. They are often classified by their action on blood agar. One group,
the alpha-hemolytic species, reduce hemoglobin and leave a greenish ring in the
agar surrounding colonies. Beta-hemolytic species leave a clear zone. Others
are nonhemolytic.
Pathogenic
streptococci have the ability to destroy phagocytic cells and produce enzymes
that break down the connective tissue of the host, allowing the infection to
spread as well as destroying tissue. Other enzymes dissolve blood clots, also
allowing spread of the organisms.
1) Streptococcus pyogenes—principal pathogen; causes “strep” throat,
and some strains carry a plasmid that produces the toxin of scarlet fever. Also
causes impetigo and rheumatic fever. Several strains of S. pyogenes are
the "flesh-eating" bacteria.
2) Streptococcus mutans—attaches to teeth
and is a major factor in tooth decay
3) Streptococcus pneumoniae ---usually found in pairs instead of
chains and causes pneumonia. In recent years, a strain which produces a very
rapidly-developing fatal pneumonia and is antibiotic-resistant has been seen.
4) A few are nonpathogenic and
involved in production of dairy products
c. Enterococcus—live
in areas of the body that are rich in nutrients but low in oxygen—gastrointestinal
tract, vagina, mouth. Often they are contaminants in hospitals and cause nosocomial infections, especially infections of surgical
wounds and the urinary tract.
1) Enterococcus
faecalis
2) Enterococcus
faecium
d. Listeria monocytogenes occasionally contaminates food, especially
dairy products. It has the ability to grow at refrigerator temperatures, and
there have been recent outbreaks of the disease listeriosis
traced to hotdogs and luncheon meats. The disease is particularly serious in
pregnant women, since it can cause severe damage to the fetus. Listeria can survive inside phagocytic
cells.
5. MYCOPLASMATALES---these
are highly pleomorphic and lack a cell wall. (They
really don’t have a gram-reaction, but are placed in the gram-positive group
anyway.) When first discovered, this group could not be grown on artificial
media, but after the discovery that media must contain sterols and meet some
other unusual requirements that is no longer true.
a. Mycoplasma---most are aerobic or facultative anaerobes. The cells are very
small (0.1 to .025 mm), which
puts them at or below the limits of resolution of a light microscope. Even
their colonies are so tiny that a magnifying glass is used to observe them.
Because their plasma membranes must be stronger than those of other bacteria,
they must incorporate sterols in the membrane, so their growth media must
include sterols, which most bacteriological media do not. They also are grown
in tissue cultures. Mycoplasma pneumoniae—causes atypical primary pneumonia (walking pneumonia)
b. Spiroplasma species, which are pathogens of plants and insects that feed on
plants. Cells have a corkscrew shape.
c. Ureaplasma species—sometimes associated with urinary tract infections. They
produce an enzyme that can split urea.
B.
HIGH G +
C GRAM-POSITIVE BACTERIA—these belong to the phylum Actinobacteria. Many are pleomorphic
and several are filamentous.
1.
Mycobacterium--these are aerobic, non-endospore-forming rods that occasionally exhibit
filamentous growth. They have a distinctive cell wall, which is similar in some
ways to that of gram-negative bacteria, although they usually stain
gram-positive. Instead of a true outer membrane, they deposit a thick waxy
layer of mycolic acid, which gives them great
resistance to drying and also prevents many antibiotics and disinfectants from
entering the cell. Nutrients also enter the cell slowly, which results in
unusually slow growth. It is this layer of mycolic
acid which makes these bacteria stain acid-fast. Most species live in soil and
water and only occasionally are pathogenic
a. Mycobacterium tuberculosis—tuberculosis
b. Mycobacterium leprae—leprosy
2.
Corynebacterium---Corynebacterium diphtheriae—causative agent of diphtheria.
Organisms in this genus tend to be pleomorphic.
3. Propionibacterium--Propionibacterium acnes—commonly associated with human acne. Other species are used in
production of Swiss cheese.
4. Gardnerella--Gardnerella vaginalis is a difficult organism to classify. It has a variable
gram-staining reaction and highly pleomorphic form.
Although its cell wall has a gram-positive structure, it is very thin and often
stains gram-negative. Whatever its classification is causes the most common
form of bacterial vaginitis. (Yeast infections are
much more common, but the cause of these is a fungus.)
5. Frankia—causes nitrogen-fixing
nodules to form in alder tree roots
6. Streptomyces—strict aerobes which
are responsible for producing many of our antibiotics. They reproduce by
forming asexual conidiospores at the ends of their
filaments. They have a great ability to produce extracellular enzymes that
allow them to use nutrients found in soil.
Many of our antibiotics have come from various species of this genus.
7. Actinomyces—these facultative
anaerobes are found in the mouth and throat of humans and animals. They may
form filaments. Actinomyces israelii
causes actinomycosis, a disease that attacks tissues
of the head, neck or lungs.
8. Nocardia--this is a group of bacteria which are most frequently found
associated with soil. Most of them are nonpathogenic. They are aerobic and
reproduce by forming filaments which fragment into small rods. They have a cell
wall which resembles that of the mycobacteria, so
they often stain acid-fast. Nocardia asteroides—occasionally causes a chronic pulmonary
infection which is difficult to treat. This organism can also cause a condition
called mycetoma, a destructive infection of the feet
or hands.
A.
Chlamydia & Chlamydophila—these are
tiny, gram-negative, coccoid bacteria. They are
obligate intracellular parasites with complicated life cycles. Reproduction must
occur within the host cell and this group also must use the host cell's
ATP. They are transmitted by personal
contact or aerosols. In the lab, they must be grown in tissue cultures or other
living cells.
1. Chlamydia trachomatis—this organism
causes:
·
A serious eye infection
called trachoma, which was a common cause of blindness before the age of
antibiotics, and still causes blindness in undeveloped countries.